With the increase in requirement for the display quality of a liquid crystal display device, liquid crystal display elements are being developed toward high performance. Liquid crystal displays have been developed from TN, STN to IPS, VA, PSA, and other types. Based on different displaying principles, different pretilt angles are required for a variety of the liquid crystal displays. However, in order to obtain better display quality, one common requirement for various types of the liquid crystal display devices is to have good liquid crystal alignment property. Therefore, the liquid crystal alignment property of a liquid crystal alignment agent is an important research subject affecting the display quality of the liquid crystal display devices.
JP H09-176651 discloses a liquid crystal alignment agent used for forming a liquid crystal alignment film having good pretilt angle stability. The liquid crystal alignment agent includes a polymer obtained by subjecting a tetracarboxylic dianhydride compound having a steroid skeleton and a diamine compound having a steroid skeleton to a reaction. The liquid crystal alignment film can be maintained at a certain pretilt angle under high temperature to achieve a good liquid crystal alignment property. However, the liquid crystal alignment film has a problem of high ion density, especially after a period of exposure to ultraviolet (UV) irradiation, and therefore cannot be accepted by the industry.
There is an increasing demand in the art for a liquid crystal alignment agent to have superior UV stability so that a low ion density of a liquid crystal display element employing a liquid crystal alignment film formed from the liquid crystal alignment agent can be maintained even after long-term UV irradiation.